Overload management is of vital importance in wide-area publish/subscribe systems, yet current solutions are best-effort. In this paper, we present an admission control scheme for overload management in large-scale and scalable content-based publish/subscribe systems. We analyze the stumbling block for implementing admission control in publish/subscribe systems, and point out how it differs from admission control schemes in other research areas. We propose a cover relation based algorithm to compute subscription resource requirements and an admission control algorithm based on subscription routing. The scheme ensures time, space and flows decoupling without sacrificing scalability of publish/subscribe systems. Finally, we conduct experiments to verify the effectiveness of the scheme.